Are Cenozoic Topaz Rhyolites the Erupted Equivalents of Proterozoic Rapakivi Granites? Examples from the Western United States and Finland ⁎ Eric H

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Lithos 97 (2007) 219–246 www.elsevier.com/locate/lithos

Are Cenozoic topaz rhyolites the erupted equivalents of Proterozoic rapakivi granites? Examples from the western United States and Finland ⁎ Eric H. Christiansen a, , Ilmari Haapala b, Garret L. Hart c

a Department of Geological Sciences, Brigham Young University, Provo, Utah 84602, USA b Department of Geology, P.O. Box 64, FIN-00014, University of Helsinki, Finland c School of Earth and Environmental Sciences, Washington State University, Pullman, WA 99164-2812, USA

Received 6 December 2005; accepted 25 January 2007 Available online 2 February 2007

Abstract

Eruptions of topaz rhyolites are a distinctive part of the late Cenozoic magmatic history of western North America. As many as 30 different eruptive centers have been identified in the western United States that range in age from 50 to 0.06 Ma. These rhyolite lavas are characteristically enriched in fluorine (0.2 to 2 wt.% in glass) and lithophile trace elements, such as Be, Li, Rb, Cs, Ga, Y, Nb, and Ta. REE patterns are typically flat with large negative Eu anomalies; negative Nb–Ta anomalies are small or nonexistent; and F/Cl ratios in glasses are high (N3). These features, together with high Fe/Mg ratios and usually low f O2, set them apart from subduction-related (I-type) silicic rocks. The rhyolites are metaluminous to only slightly peraluminous, lack indicator minerals of strongly peraluminous magmas, and have low P and B contents; these features set them apart from S-type silicic magmas. Instead, topaz rhyolites have the major and trace element, mineralogic, and isotopic characteristics of aluminous A-type or within-plate granites. Topaz rhyolites were formed during regional extension, lithospheric thinning, and high heat flow. Topaz rhyolites of the western United States crystallized under subsolvus conditions, and have quartz, sanidine, and Na- plagioclase as the principal phenocrysts. Fluorite is a common magmatic accessory, but magmatic topaz occurs only in a few complexes; both are mineralogical indicators of F-enrichment. Many also crystallized at relatively low f O2 (near QFM) and contain mafic silicate minerals with high Fe/(Fe+Mg) ratios. Some crystallized at higher oxygen fugacities and are dominated by magnetite and have titanite as an accessory mineral. Post-eruption vapor-phase minerals include topaz, garnet, red Fe–Mn-rich beryl, bixbyite, pseudobrookite, and hematite. They are genetically related to deposits of Be, Mo, F, U, and Sn. Topaz rhyolites erupted contemporaneously with a variety of other igneous rocks, but most typically they form bimodal associations with basalt or basaltic andesite and are unrelated to large collapse calderas. In their composition and mineralogy, topaz rhyolites are similar to the evolved members of rapakivi granite complexes, especially those of Proterozoic age in southern Finland. This suggests similarity in origin and lessons learned from these rocks may help us better understand the origins of their more ancient counterparts. For example, all topaz rhyolites in western North America seem to be intrinsically related to extension following a regional period of subduction-related volcanism. Cratonized Precambrian crust is found beneath almost all of them as well. Trace element models, Sr–Nd isotopic data, and geologic associations indicate that topaz rhyolites probably form by fractional crystallization of silicic magma which originated by small degrees of melting of hybridized continental crust containing a significant juvenile mantle component not derived from a subduction zone (i.e., intrusions of within-plate mafic magma). The Sr and Nd isotopic compositions of the topaz rhyolites lie between the fields of contemporaneous mafic magmas and older calc-alkaline dacites and rhyolites. Intraplate mafic magmas and their derivatives appear

⁎ Corresponding author. Fax: +1 801 422 0267. E-mail address: [email protected] (E.H. Christiansen).

0024-4937/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2007.01.010 220 E.H. Christiansen et al. / Lithos 97 (2007) 219–246 to have lodged in the crust and were then re-melted by subsequent injections of mafic magma. In turn, the mafic mantle-derived magma probably formed as a result of decompression related to lithospheric extension or to convective-flow driven by the foundering of a subducting lithospheric plate. Although significant uncertainty remains, we suggest that topaz rhyolites (and by extension rapakivi granites) are probably not simply melts of mid-crustal granodiorites, nor are they derived solely from felsic crust that was previously dehydrated or from which melt had been extracted as proposed in earlier papers. © 2007 Elsevier B.V. All rights reserved.

Keywords: Topaz rhyolite; Cenozoic; Rapakivi granite; Proterozoic; Fluorine; A-type; Anorogenic

1. Introduction mass spectrometer (TIMS) at the University of Wiscon- sin–Madison Radiogenic Isotope Laboratory following Topaz in rhyolitic lavas was first discovered in 1859 in standard procedures (e.g., Johnson and Thompson, western Utah and reported in the scientific literature by 1991). Samples were crushed in a steel jaw crusher and Simpson (1876). Since then, topaz-bearing rhyolitic lavas powdered in an agate ball mill. For Sr and Nd, 50 mg have been identified in much of western United States aliquots of whole-rock powders were spiked with 84Sr- (Christiansen et al., 1983), Mexico (Huspeni et al., 1984), and 150Nd-enriched tracers and dissolved in a mixture of the Yukon Territory of Canada (Sinclair, 1986), eastern HF and HNO3, the elements were then separated using Russia and Mongolia (Kovalenko and Kovalenko, 1984). standard ion-exchange chromatography. Total procedur- Topaz-bearing rhyolite dikes of Proterozoic age have also al blanks were b0.1 ng for both Sr and Nd, which are been found in southern Finland (Haapala, 1977)and negligible. For Pb isotope ratios, 100 mg aliquots of central Arizona (Kortemeier and Burt, 1988). Christian- whole-rock powder were dissolved in a mixture of HF sen et al. (1986) concluded that topaz rhyolite lavas from and HNO3, and Pb was separated using HBr and HCl on the western United States are generally similar to some an ion-exchange column. Total procedural blanks for Pb A-type granites. This report summarizes the character- were also negligible at b2 ng. Both Sr and Nd isotope istics of Cenozoic topaz rhyolites–emphasizing new data compositions were exponentially corrected for mass from the Wah Wah Mountains of southwestern Utah–and fractionation using 86Sr/88Sr =0.1194 and 146Nd/144Nd= compares them to Proterozoic rapakivi granites of the 0.7219, respectively. Within-run errors in measured Fennoscandian shield (Fig. 1). Southern Finland is the 87Sr/86Sr ratios for dynamic analyses are determined as type locality of these unique anorogenic granitic rocks, ±2 standard error (2SE) using n=120 (number of mea- which Haapala and Rämö (1992) have redefined as sured ratios). The 87Sr/86Sr ratio measured for NBS-987 A-type granites with rapakivi texture. Once the similar- during this study was 0.710265±8 (2SE, n =13). ities between Cenozoic topaz rhyolites and Proterozoic Neodymium was measured as NdO+ and is presented as rapakivi granites are clear, we consider a new model for εNd values relative to present day CHUR, taken to be the origin of topaz rhyolites and its implications for the equal to BCR-1, measured during the analytical session as petrogenesis of rapakivi granites in particular and A-type 0.512636±5 (2SE). Within-run errors in measured granites in general. 143Nd/144Nd ratios for dynamic analyses are reported as 2SE where n=150 (number of measured ratios). Twelve 2. Methods of study analyses of an internal Ames Nd standard yielded a 143Nd/144Nd ratio and precision of 0.511977±3 (2SE). New geochemical data in this paper are presented for Lead isotope ratios were corrected for mass fractionation rhyolite lavas from in and near the Wah Wah Mountains by +0.14% per atomic mass unit based on fourteen of southwestern Utah (Fig. 2). Major and trace element analyses of NBS-981 (±0.005%; 2SE) and NBS-982 compositions were collected by X-ray fluorescence (±0.008%; 2SE) standards. spectrometry at Brigham Young University. Analyses of international materials for 31 elements can be found at 3. Distribution and ages http://www.geology.byu.edu/faculty/ehc/.Elemental and isotopic compositions of other topaz rhyolites and 3.1. Distribution of topaz rhyolites in western United for Finnish rapakivi granites are taken from the States references cited below. New Sr, Nd, and Pb isotope compositions were mea- Topaz rhyolites are widespread in western North sured on a GV Instruments Sector 54 thermal ionization America (Fig. 1) and have been found as far north as E.H. Christiansen et al. / Lithos 97 (2007) 219–246 221

Fig. 1. (a) Distribution of topaz rhyolites (filled circles) from western United States. All are found within the extensional terranes of the Basin and Range Province and the Rio Grande Rift. Modified from John et al. (2000) and Dickinson (2002). The 87Sr/86Sr line marks the western edge of Precambrian basement (modified from Kistler and Peterman, 1978; Wooden et al., 1998; Tosdal et al., 2000). (b) Precambrian rapakivi granites of southern Finland (modified from Lukkari, 2002; Haapala and Lukkari, 2005). (c) Index map showing location of (b) in Fennoscandia.

Montana and extend southward into central Mexico. Most (Castor and Henry, 2000) is the only one that has been known topaz rhyolites in the western United States lie found in the northwestern Basin and Range province, in within the eastern and southern Basin and Range province spite of common bimodal (basalt–rhyolite) volcanism and and along the Rio Grande rift and thus appear to surround extensional faulting throughout the province. The young the Colorado Plateau. Nearly all topaz rhyolites lie east of lithosphere in this region, with its mafic crust, does not the initial 87Sr/86Sr=0.706 line as determined for generally appear to have a composition appropriate for the Mesozoic plutonic rocks (Fig. 1, Kistler and Peterman, generation of topaz rhyolites. The distribution of topaz 1978; Wooden et al., 1998; Tosdal et al., 2000). This line rhyolites is entirely included in the region of Cenozoic is taken by these investigators to mark the westernmost extensional faulting (Fig. 1). Their emplacement appears extent of the Precambrian craton in the western United to have spanned most of the Cenozoic Era with isotopic States. To the west is a series of allochthonous or accreted ages ranging from 50 Ma (Little Belt Mountains of terranes composed of ocean-floor or island arc crust (e.g., Montana) to 0.06 Ma (Blackfoot lava field of southern Speed, 1979). These terranes may have formed as oceanic Idaho), although all but 3 are younger than 30 Ma. crust at the margin of North America during the Paleozoic In the Wah Wah Mountains and vicinity of south- and early Mesozoic Eras and were later accreted (Oldow, western Utah (Fig. 2), which are considered in more 1984). The topaz rhyolite lava at Buff Peak, Nevada detail in this paper, there were two episodes of topaz 222 E.H. Christiansen et al. / Lithos 97 (2007) 219–246

Fig. 2. Simplified geologic map of southwestern Utah showing the distribution of topaz-bearing rhyolites of two different ages included in the Steamboat Mountain Formation and the Blawn Formation. These rhyolites erupted across the Oligocene calc-alkaline andesite to rhyolite suite centered on the Indian Peak caldera. North-trending ranges are bounded by buried Miocene and younger normal faults. Modified from Best et al. (1987). rhyolite volcanism—one at 22 to 18 Ma and a second at 3.2. Distribution of Fennoscandian rapakivi granites about 13 to 11 Ma (Thompson, 2002). Fission track, structural, and stratigraphic studies suggest that exten- The rapakivi granites of Fennoscandia form large sion in the eastern Great Basin began about 22 to 17 Ma composite batholiths and smaller satellitic stocks across (Rowley et al., 1978; Stockli et al., 2001; Dickinson, central Sweden, southern Finland, into Russia, beneath 2002) and eventually formed a series of north-trending the floor of the Baltic Sea and in the Baltic countries horsts and grabens (Fig. 2). Thus the onset of extension (Fig. 1b). The rapakivi granites and associated mafic is closely tied to the eruption of the oldest topaz rhyo- rocks can be divided from east to west into four area- lites in this area. constrained age groups: the Salmi batholith in Russian E.H. Christiansen et al. / Lithos 97 (2007) 219–246 223

Karelia (1.55 to 1.53 Ga), the Wiborg batholith and Keith et al. (1994) describe a graben into which a satellites in southwestern Finland and Estonia (1.67 to topaz–beryl-bearing rhyolite erupted 22 Ma, suggest- 1.62 Ga), the rapakivi batholiths and satellites in south- ing that extension may have begun just prior to the western Finland and Latvia (1.59 to 1.54 Ga), and the eruption of the rhyolites. Pronounced extension rapakivi–gabbro complexes of central Sweden (1.53 to began in the region sometime between 22 Ma 1.47 Ga) (e.g., Rämö et al., 2000; Haapala et al., 2005). (Rowley et al., 1978) and 17 Ma (Stocklietal., A variety of tectonic environments has been proposed 2001) and eventually formed the present system of for the generation of the Fennoscandian rapakivi gran- horsts and grabens (Fig. 2). ites, but an incipient extensional setting is indicated by Indeed, extensional tectonism appears to be the com- oriented swarms of mafic dikes, shallow grabens imaged mon factor in almost all areas where topaz rhyolites geophysically, and thinning of the crust across the erupted in the western United States. Episodes of topaz region (Haapala et al., 2005). The rapakivi granites of rhyolite magmatism coincide with periods of lithospher- Fennoscandia were emplaced in Proterozoic crust that is ic extension: 1) in the eastern Great Basin where normal a few hundreds of millions of years older than the faulting may have begun as early as 22 Ma ago as noted granites themselves (Rämö and Haapala, 1995). above and then was renewed under a different stress orientation about 14 Ma which has persisted to the 4. Magma–tectonic associations present (Zoback et al., 1981); 2) along the northern Nevada rift that opened 16 Ma (Stewart et al., 1975; The nature of igneous rock associations and con- Zoback and Thompson, 1978; John et al., 2000); 3) in temporaneous tectonic activity give some clues about Montana where normal faulting began about 40 Ma ago the generation of magmas. This is especially important (Chadwick, 1978) and intra- or back-arc graben forma- for Cenozoic topaz rhyolites of the western United tion may have begun as early as 50 Ma (Armstrong, States, where we can compare young igneous rocks of 1978); 4) along the Rio Grande rift and its northern “known” tectonic setting with the much older, but extension into Colorado which initially developed about geochemically similar, rapakivi granites. 30 Ma ago (Eaton, 1979); and 5) in western Arizona where detachment faulting and crustal extension were 4.1. Magmatic associations for topaz rhyolites underway by before 15 Ma (Suneson and Lucchitta, 1983). The intimate association of extensional tectonics In the Wah Wah Mountains (Fig. 2), for example, and topaz rhyolite magmatism in the western United following a Cretaceous episode of folding and thrust States implies a strong genetic connection. faulting, subduction-related calc-alkaline volcanism The magmatic associations of topaz rhyolites may began in the early Oligocene (about 32 Ma) and con- also place important constraints on their origins. As in tinued until the early Miocene (Best et al., 1989). This the Wah Wah Mountains, topaz rhyolite magmatism volcanism produced widely scattered, partly clustered, consistently follows a slightly older episode of subduc- composite volcanoes with andesitic to dacitic lava tion-related calc-alkaline magmatism (magnesian in the flows as well as small volumes of isolated andesitic sense of Frost et al., 2001a). Lipman et al. (1972) and lavas. Widespread dacitic to rhyolitic ash flows erupted Christiansen and Lipman (1972) first concluded that the from large collapse calderas in the Wah Wah Moun- Cenozoic magma–tectonic evolution of the western tains and Indian Peak Range and other nearby ranges United States may be divided into two fundamentally (Fig. 2). Dacite gradually gave way to high K2O different stages. An early suite of calc-alkaline magmas trachydacite ignimbrites by about 26 Ma. Following a was associated with subduction of the Farallon plate. local cessation of volcanism, the older topaz rhyolite Eruptions of andesitic lavas, dacites, and rhyolites were domes(BlawnFormation,22to18Ma)eruptedalong common, many of which are potassium rich. The silicic with trachyandesite lava flows (62 to 54% SiO2)to magmas were erupted mostly as ash flows associated form a bimodal suite (see Fig. 5). A Miocene lull in with caldera collapse and many of the andesites formed volcanic activity in the eastern Great Basin was large fields of coalesced stratovolcanoes. This magma- followed by renewed bimodal magmatism that began tism swept southward across much of the western about 13 Ma in and near the Wah Wah Mountains United States. It started in Montana about 50 million (Steamboat Mountain Formation; Best et al., 1987; years ago and moved southward and then stagnated in Fig. 2). Topaz-bearing rhyolites were again accompa- southern Nevada and Utah between about 30 and 25 Ma. nied by the eruption of trachybasalts to trachyandesites At the same time, similar magmas were also erupted in the Wah Wah Mountains. across southern Arizona and New Mexico as well as 224 E.H. Christiansen et al. / Lithos 97 (2007) 219–246 throughout northwestern Mexico. In all of these areas, subduction zone magmatism was replaced by a bimodal suite of mafic magmas and rhyolite. The timing of the switch is not the same across the entire region. Ultimately, the younger magmatism became associated with lithospheric extension and normal faulting. In many places, the transition was marked by a magmatic gap of several million years. In other cases, the transition was gradual. For example, the early Miocene rhyolites of the Wah Wah and Needle Ranges (Fig. 2) form a bimodal (trachyandesite–rhyolite) association Fig. 3. Synthetic cross section of a topaz-bearing rhyolite dome and flow. The details are modeled after rhyolite lavas in the Wah Wah locally, but contemporaneous volcanism elsewhere in Mountains of southern Utah (Christiansen, 1980; Christiansen et al., southwestern Utah was still broadly calc-alkaline and 1996, Thompson, 2002). A typical dome is 0.5 to 3 km across and a continuous from andesite to rhyolite. However, by the few 100 m high. Flows may extend for a few kilometers away from the time the younger topaz rhyolites erupted in the Wah vent. Wah Mountains, calc-alkaline andesite to rhyolite magmatism had ceased throughout the region. The mafic consisting of stratified pyroclastic-surge units produced members of the younger bimodal suites varied widely during pulsing unsteady eruptions. Some short and thin and included potassic trachybasalt and trachyandesite as (less than 1 m) lithic-rich ash flows probably resulted from well as alkaline and tholeiitic basalt. Topaz rhyolites at minor collapse of low eruption columns. Mantling ash- Kane Springs Wash volcanic center in southern Nevada fall units punctuate the record of explosive volcanism at are associated with contemporaneous peralkaline tra- several localities. Vitrophyres a few meters thick are chytes and rhyolites. present at the bases of some lava flows that overlie the pyroclastic deposits. Others have basal flow breccias 4.2. Magmatic associations for rapakivi granites (about 1 m thick) produced as the flow front crumbled, slumped, and was overridden by the advancing flow. Magmas associated with Finnish rapakivi complexes Rapidly quenched vitrophyric blocks from the flow front were likewise diverse, but generally were not calc- are common in this part of the flow (Fig. 3). In the upper alkaline. Rapakivi granites in Finland are part of a portions of the flows, felsitic, flow-layered lavas with bimodal sequence that includes tholeiitic gabbros (as abundant vapor-phase cavities are typical. dikes, sills, and small screens between intrusions), These features suggest that the pyroclastic eruptions norites, anorthosites, and ferrodiorites (Rämö, 1991; were initiated as rising magmas explosively mixed with Salonsaari, 1995). The silicic members include various groundwater (hydromagmatic eruptions). Once the vent petrographic types of granite, rhyolite (quartz porphyry) was cleared, relatively quiet eruption of rhyolite lava dikes and local lavas, as well as rare syenite dikes. proceeded. The transition from pyroclastic to lava eruptions may also correlate with the eruptive degassing of 5. Mode of emplacement the magma or with the evisceration of a volatile-rich cap to a small magma chamber (Byrd and Nash, 1993). 5.1. Emplacement of topaz rhyolites The volume of magma in individual domes or flows ranges from less than 0.2 km3 to a probable maximum of The eruption and emplacement of the topaz rhyolites about 10 km3. However, fairly large volumes (10 to of the Wah Wah Mountains were typical of most others in 50 km3) of coalesced domes and flows accumulated the western United States. The rhyolites of both erup- over short (about 1 m.y.) time intervals in the Wah Wah tive episodes occur as isolated intrusive plugs without Mountains and Thomas Range of western Utah, and in significant pyroclastic deposits, as isolated endogenous New Mexico's Black Range (Duffield and Dalrymple, lava domes or flows with underlying pyroclastic breccias 1990). It is important to contrast both the small volumes and tuffs and as groups of coalesced domes and flows with and mode of emplacement of these F-rich magmas with interlayered tephra deposits (Fig. 3). Vent-clearing other Cenozoic rhyolites from the same region which explosions locally created breccias that underlie tuffs. generally erupted from large collapse calderas (e.g., These near vent explosion breccias contain abundant Indian Peak, Central Nevada, and San Juan caldera lithic inclusions of the local country rocks. The explosion complexes, the Snake River Plain, or the southwest breccia is commonly overlain by remnants of a tuff ring Nevada volcanic field). Dacitic and rhyolitic ash flows E.H. Christiansen et al. / Lithos 97 (2007) 219–246 225 from these calderas have volumes 1 to 3 orders of 6. Petrography and mineralogy magnitude larger (e.g., Smith, 1979; Best et al., 1989). On the other hand, the eruption of large volumes of F-rich 6.1. Mineralogy of topaz rhyolites magma over geologically short time intervals in the Thomas Range, Utah, and in the Black Range, New Topaz rhyolites are generally flow-layered and nearly Mexico, suggests that some topaz rhyolites may emanate aphyric to sparsely crystalline, but a few contain as from magma chambers with volumes approaching those much as 40% phenocrysts. The major phenocrysts in of caldera-related plutons. Only one Cenozoic topaz- topaz rhyolites in the Wah Wah Mountains are typical of bearing granite has been found in the western United others and include sanidine, smoky quartz, sodic plagio- States—the 21 Ma Sheeprock granite of western Utah clase, and sparse Fe-rich biotite, in order of abundance. (Christiansen et al., 1988; Richardson, 2004). The pluton Magmatic topaz has not yet been found in the Wah Wah has all of the geochemical characteristics of topaz rhyolites. In the western United States it has only been rhyolites from the eastern Great Basin. It was emplaced identified in the Honeycomb Hills complex (Congdon at a shallow level and covers an area of about 20 km2. and Nash, 1991). Sanidine in topaz rhyolites is generally Calzia and Rämö (2005) have identified two A-type Or40 to Or60 and plagioclase is typically sodic oligo- granite plutons from the Death Valley region. Both are clase. Biotite generally has high Fe/(Fe+Mg) (Fig. 4) Miocene in age (12.4 and 10 Ma) and display rapakivi reflecting the high Fe/(Fe+Mg) of the magma (in many textures. However, compared to contemporaneous topaz cases, molar Mn and Ti exceed Mg), and the prevalence rhyolites, they are more oxidized, less enriched in incom- of relatively low f O2 during crystallization. At compa- patible trace elements (e.g., F, Rb, HREE), and lack topaz. rable Fe/(Fe+Mg) ratios, the Altot in biotites from topaz rhyolites is less than from strongly peraluminous two- 5.2. Emplacement of rapakivi granites mica granites (Fig. 4). Biotites from topaz rhyolites also have high F-contents (up to 5 wt.%). Fluorine concen- Rapakivi granites in Fennoscandia typically occur as trations this high for Fe-rich biotites suggest crystalli- sharply discordant, composite high-level batholiths and zation at high f HF and f HF/f H2O. F/Cl ratios in the stocks that intrude metamorphic bedrock. In southern biotites also suggest crystallization at high f HF/f HCl Finland, the largest is the Wiborg batholith which is (e.g., Byrd and Nash, 1993). Rare phenocryst phases almost 200 km across. The batholiths may be sheetlike include clinopyroxene (in high T, F-poor magmas bodies 5 to 10 km thick (Laurén, 1970; Haapala and from the Thomas Range, Utah, and Jarbidge, Nevada, Rämö, 1992; Korja et al., 1993). Individual plutons are rhyolites), fayalite (Kane Springs Wash, Nevada), Fe- as small as a few kilometers across, such as the topaz- rich hornblende, or Fe–Mn garnet. Magmatic accessory bearing parts of the Eurajoki (Haapala, 1977), Artjärvi and Sääskärvi (Lukkari, 2002), Suomenniemi (Rämö, 1991), and Kymi (Haapala and Lukkari, 2005) intrusions (Fig. 2). The zoned, composite Ahvenhisto pluton is larger and covers about 240 km2 (Edén, 1991), but only a small portion of the pluton is topaz bearing. The depth of emplacement of the Finnish rapakivi granites is, thus far, poorly constrained. Elliott (2001), using amphibole compositions, calculated crystallization pressures of 3.6 to as much as 5 kb, but these were not corrected for the significant effect of low f O2.Conse- quently, these estimates are probably over-estimates (Anderson and Smith, 1995). Miarolitic cavities are present in some of the topaz-bearing phases; rapakivi-age volcanic and subvolcanic rocks probably formed much of the roof of the Wiborg batholith. Subvolcanic quartz- Fig. 4. Biotite compositions of topaz rhyolites and granites from the feldspar porphyry dikes are often associated with the western United States compared with those of rapakivi granites of plutons as are swarms of tholeiitic diabase dikes (Rämö southern Finland. Data from Haapala (1977), Christiansen et al. (1986), Rogers (1990 for the Sheeprock granite), Salonsaari and Haapala, 1995). Pegmatites are also rare. These (1995), Rieder et al. (1996), and Elliott (2001). Fields for two-mica associations point to a shallow level of emplacement for (broadly S-type) and calc-alkaline (broadly I-type) granites are the granites. included for comparison (Christiansen et al., 1986). 226

Table 1 Representative chemical compositions of topaz rhyolites from the western United States and topaz granites from southern Finland ⁎ Reference SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2OK2OP2O5 Total LOI Anal total ..Crsine ta./Lto 7(07 219 (2007) 97 Lithos / al. et Christiansen E.H. Topaz Rhyolites, western United States Blawn Formation (22 to 18 Ma) LAM-1-38-2 77.06 0.07 12.50 1.14 0.13 0.10 0.61 3.66 4.73 0.01 100.00 1.02 99.96 TET-9-43-2 77.01 0.09 12.44 0.94 0.11 0.10 0.71 3.91 4.67 0.03 100.00 0.76 99.86 RED-04 Thompson (2002) 76.88 0.05 12.91 0.98 0.13 0.07 0.34 3.15 5.49 0.01 100.00 3.58 99.42 RED-06 Thompson (2002) 77.89 0.10 11.87 1.06 0.09 0.26 0.63 3.12 4.98 0.00 100.00 0.74 99.16 RED-12 Thompson (2002) 77.44 0.10 11.96 1.07 0.05 0.32 0.58 3.02 5.45 0.01 100.00 0.84 99.87 RED-33 Thompson (2002) 76.85 0.05 12.72 1.08 0.10 0.02 0.40 4.15 4.62 0.01 100.00 0.51 99.22 RED-34 Thompson (2002) 76.73 0.04 12.88 1.09 0.11 0.03 0.32 4.19 4.61 0.00 100.00 0.37 99.04 RED-40 Thompson (2002) 76.43 0.04 13.02 1.08 0.09 0.04 0.44 4.13 4.73 0.00 100.00 0.42 98.07 RED-41 Thompson (2002) 77.58 0.06 13.06 0.34 0.01 0.15 0.43 3.39 4.97 0.00 100.00 1.12 98.96 Steamboat Formation (13 to 11 Ma) BBS-8-229-2 77.49 0.08 12.36 1.07 0.08 0.13 0.25 3.75 4.76 0.01 100.00 0.54 100.12 LAM-9-103-2 75.77 0.07 13.18 1.07 0.11 0.11 0.69 4.06 4.90 0.05 100.00 1.05 99.60 WW-8014 76.41 0.08 12.20 1.23 0.08 0.11 0.99 4.16 4.69 0.05 100.00 99.40 –

WW-8016 76.30 0.04 12.70 1.14 0.12 0.00 0.42 4.71 4.57 0.00 100.00 99.09 246 WW-8018 76.29 0.04 12.76 1.16 0.07 0.03 0.47 4.67 4.51 0.00 100.00 99.04 WW-8020 76.45 0.09 12.60 1.26 0.08 0.08 0.46 3.97 5.01 0.00 100.00 99.21 WW-8029 76.52 0.04 12.75 1.16 0.13 0.00 0.31 3.74 5.35 0.00 100.00 99.00 WW-8011A 76.87 0.05 12.77 0.67 0.05 0.00 0.37 4.57 4.65 0.00 100.00 98.83 WW-8011B 76.35 0.08 12.74 1.11 0.10 0.00 0.43 4.49 4.70 0.00 100.00 99.24 Spor Mountain Rhyolite (21 Ma) Christiansen et al. (1986) 75.25 0.05 13.58 1.45 0.06 0.16 0.65 3.75 5.04 0.00 100.00 99.11

Topaz granites associated with rapakivi granites, Finland Suomenniemi-4 Rämö and Haapala (1995) 75.40 0.10 13.52 1.36 0.03 0.09 0.83 3.61 5.03 0.03 100.00 0.53 99.66 Eurajoki-2 Rämö and Haapala (1995) 75.99 0.06 13.86 1.45 0.04 0.02 0.87 3.32 4.33 0.06 100.00 0.50 100.25 Eurajoki 5/IH/2001 Haapala et al. (2005) 76.48 0.02 13.75 0.92 0.05 0.00 0.65 3.73 4.39 0.02 100.00 0.24 99.93 Saaskjarvi-17E Lukkari (2002) 74.38 0.20 13.45 2.14 0.01 0.21 0.92 3.01 5.58 0.10 100.00 0.50 100.30 Ahvenisto Edén (1991) 75.14 0.02 14.25 2.00 0.04 0.01 0.71 2.86 4.96 0.01 100.00 99.06 Kymi Haapala et al. (2005) 74.29 0.02 15.24 0.87 0.02 0.10 0.77 4.09 4.59 0.01 100.00 0.40 100.15 F Cl Sc V Cr Ni Zn Ga Rb Sr Y Zr Nb Ba La Ce Nd Sm Pb Th U Topaz Rhyolites, western United States Blawn Formation (22 to 18 Ma) LAM-1-38-2 2900 100 1 2 3 6 113 24 626 6 85 156 117 25 47 91 43 12 49 62 16 TET-9-43-2 5100 100 3 3 2 7 78 25 636 7 52 155 149 30 43 78 31 10 48 72 22 RED-04 2883 778 1 2 4 6 90 26 813 2 82 159 150 17 34 84 38 9 51 49 30 RED-06 4038 79 3 6 3 8 53 20 446 8 73 158 83 10 63 116 53 12 34 63 11 RED-12 3660 79 3 5 3 11 40 20 477 12 64 157 82 10 52 96 44 11 37 61 10 RED-33 5240 117 3 5 2 9 52 26 709 3 90 148 136 7 50 89 41 10 42 54 15 RED-34 4708 77 1 4 2 7 83 27 789 2 74 136 152 3 57 87 47 10 50 49 12 RED-40 3938 77 2 2 – 9 74 27 796 5 94 144 153 23 38 71 31 8 51 51 19 RED-41 1845 143 2 1 2 14 108 26 641 14 59 162 153 9 34 30 25 7 44 27 13 Steamboat Formation (13 to 11 Ma) BBS-8-229-2 2000 60 2 2 1 6 43 23 553 4 79 147 90 18 40 95 39 9 48 57 17 219 (2007) 97 Lithos / al. et Christiansen E.H. LAM-9-103-2 900 100 2 4 1 7 20 29 780 12 31 111 139 52 37 92 37 9 56 60 20 WW-8014 5804 59 1 7 6 3 46 21 522 8 112 170 86 12 46 110 45 13 40 62 16 WW-8016 5720 97 2 3 6 1 79 28 727 2 131 164 122 18 32 80 39 11 58 70 32 WW-8018 5461 113 0 3 5 1 44 27 679 3 136 177 126 20 31 81 37 9 44 64 18 WW-8020 4621 90 1 8 6 1 65 22 511 3 104 168 81 17 72 122 52 15 51 61 15 WW-8029 4172 1721 0 5 6 0 105 27 715 0 128 173 124 10 31 81 37 9 57 70 28 WW-8011A 4900 78 1 8 6 1 46 25 671 3 90 172 143 16 32 81 38 9 40 70 21 WW-8011B 3845 1238 1 5 5 1 77 23 521 24 93 160 91 155 41 91 37 10 49 55 21 Spor Mountain Rhyolite (21 Ma) 10205 609 3 7 6 2 80 36 1048 8 118 121 131 21 59 142 63 15 43 76 31

Topaz granites associated with rapakivi granites, Finland Suomenniemi-4 7100 4 47 24 567 21 116 118 51 163 161 Eurajoki-2 10400 74 965 10 70 60 150 40 91 34 9 30 9

Eurajoki 5/IH/2001 11900 100 12 197 60 1050 8 56 51 70 28 47 97 37 9 88 28 6 – 246 Saaskjarvi-17E 2500 195 3 2 18 48 26 412 65 40 171 22 282 49 32 12 Ahvenisto 14270 740 20 70 80 70 20 100 40 10 Kymi 14500 110 8 67 61 978 22 7 12 58 151 65 154 35 8 130 31 ⁎ Fe2O3 =Total Fe as Fe2O3. Major element concentrations normalized to 100% on a volatile-free basis. Trace element concentrations in ppm. 227 228 E.H. Christiansen et al. / Lithos 97 (2007) 219–246 minerals include zircon, thorite, uraninite, allanite, apatite, the smaller, more evolved plutons (Rämö and Haapala, fluorite, and Fe–Ti–Mn–Nb oxides. 1995). Peralkaline rocks are rare, but Rämö (1991) Mineral geothermometry indicates that most topaz described peralkaline hypersolvus syenite dikes in the rhyolites crystallized at low temperatures around 650 to Suomenniemi complex. 700 °C. Oxides reveal that f O2 was commonly low, near The youngest phases of several Finnish rapakivi the QFM oxygen buffer, although some topaz rhyolites granites are felsic, porphyritic or equigranular micro- crystallized under fairly oxidizing conditions as indi- cline-albite granites that often contain topaz and are cated by oxide and biotite compositions and the associated with greisen-type mineralization (Fig. 2). In presence of titanite in Lake City and Chalk Mountain, these topaz granites, the dark mica is F-rich siderophyllite Colorado, Mineral Range, Utah, Sheep Creek Moun- (Fig. 4). Fluorine is as high as 5 wt.% in these micas tains and Jarbidge, Nevada, rhyolites. It thus appears (Haapala, 1977; Haapala and Lukkari, 2005). Character- that there are oxidized and less oxidized topaz rhyolites, istic accessory minerals are fluorite, monazite, bastnae- analogous to Proterozoic anorogenic granites from the site, ilmenite, cassiterite, columbite, and thorite, along western United States which consist of an ilmenite- and with rare zircon, apatite, and magnetite. Miarolitic a magnetite-series (Anderson, 1983; Anderson and cavities are common and subsolidus reactions are evident Morrison, 2005). All topaz rhyolites in the western in their textures and mineral compositions (Haapala, United States are two-feldspar rhyolites, in contrast to 1977; Rämö and Haapala, 1995; Lukkari, 2002; Haapala many other bimodal rhyolites—e.g., many of the rhyo- and Lukkari, 2005). The presence of miarolitic cavities lites of the Snake River Plain, Idaho (Leeman, 1982) and and small pockets of pegmatite suggests that the topaz the peralkaline rhyolites of the western Great Basin. In rapakivi granites became water-saturated during crystal- general, one-feldspar rhyolites crystallize at higher tem- lization. Less evolved rocks probably crystallized in peratures than those inferred for topaz rhyolites. undersaturated conditions. Temperatures of crystalliza- Topaz, fluorite, alkali feldspar, spessartine garnet, tion have been estimated for the Eurajoki stock of 750 °C hematite, bixbyite, pseudobrookite, and silica minerals at 2 kb for fayalite–biotite–hornblende granite (Haapala, line gas cavities and occur in the devitrified matrix of 1977). Using amphibole-plagioclase geothermometry, some lavas. Gem-quality red beryl (up to 1 cm long) Elliott (2001) deduced temperatures averaging about occurs in one lava flow from the older episode of 725 °C for several granites from the Wiborg batholith. rhyolitic volcanism in the Wah Wah Mountains (Fig. 2), Two-feldspar temperatures in the same rocks were but is also found in the Thomas Range, Utah, and in the typically 650 °C. The Fe–Ti oxides in most rapakivi Black Range of New Mexico. Peralkaline minerals granites are dominated by ilmenite, but Kosunen (1999) (aegerine, riebeckite, etc.) are absent in the topaz-bearing has shown that the Obbnas pluton of southern Finland flows, but Nevada's Kane Springs caldera erupted has magnetite and titanite. Magnetite and titanite are also peralkaline trachyte and comendite ash-flow tuffs shortly typical accessory minerals in the 1.6 Ga anorogenic before intracaldera topaz-bearing lavas (Novak, 1984). granites of Estonia (Kirs et al., 2004).

6.2. Mineralogy of rapakivi granites 7. Geochemistry and differentiation trends

Finnish rapakivi granite plutons range from subsolvus 7.1. Geochemistry of topaz rhyolites from the Wah Wah hornblende granite to biotite granite and further to leu- Mountains cocratic topaz-bearing subsolvus granite. Some of the more mafic varieties have fayalite or an Fe-rich clino- Representative analyses of topaz rhyolites from the pyroxene. Typically, potassium feldspar is the most Wah Wah Mountains of southwestern Utah are given in abundant mineral accompanied by quartz, plagioclase Table 1, along with the composition of an average topaz (oligoclase to andesine), and Fe-rich biotite (Fig. 4). In rhyolite from Spor Mountain, central Utah. The major the biotites, Fe/(Fe+Mg) generally exceeds 0.8 and Altot element composition of topaz rhyolites from the Wah is also similar to that in biotite from topaz rhyolites Wah Mountains is fairly restricted. All are high-silica (Rieder et al., 1996). On the other hand, F in biotite is rhyolites with high Na, K, F, Fe/Mg and low Ti, Mg, Ca, comparatively low (ranging from 0.2 to 0.7 wt.%) in and P (Table 1; Fig. 5). Alkali oxides range between 8% rocks that lack topaz (Elliott, 2001). Accessory minerals and 10%. In general K2O/Na2O ratios are greater than are fluorite, zircon, apatite, ilmenite, magnetite, and one (typically about 1.2 to 1.4 by weight) but this ratio allanite or monazite. Rapakivi texture is common only in declines with differentiation. Most topaz rhyolites the larger, more mafic plutons and rare or absent in contain 12% to 14% Al2O3 (Table 1). In spite of the E.H. Christiansen et al. / Lithos 97 (2007) 219–246 229

Fig. 5. The compositions of topaz rhyolites from the western United States and Finnish rapakivi granites are similar as shown on these variation diagrams. Field for Finnish rapakivi granites from Rämö and Haapala (1995). (a) Total alkalies versus silica. Gridlines are from the IUGS chemical classification of volcanic rocks. The bimodal character of the Wah Wah volcanic rocks is evident by a gap between 63 and 75% SiO2. (b) Al-saturation index versus silica. Like rapakivi granites, most topaz rhyolite suites straddle the dividing line between metaluminous and peraluminous compositions. Alkali loss after eruption may be responsible for some of the peraluminous rocks. (c) Most topaz rhyolites are ferroan on a FeO/(FeO+ MgO) versus silica diagram. Mafic rocks from the Wah Wah Mountains are both ferroan and magnesian. Solid line is from Miyashiro (1974) and dashed line is from Frost et al. (2001a). (d) F and Cl concentrations in glassy rhyolites including topaz rhyolites. Data from Macdonald et al. (1992) on rhyolite obsidians included. continued misperception, topaz rhyolites are neither Of course, the most discriminating feature of topaz peralkaline nor strongly peraluminous (Fig. 5). The rhyolites is their high fluorine content. For Wah Wah presence of garnet and topaz (absent as vapor-phase rhyolites, fluorine concentrations in vitrophyres range minerals in peralkaline volcanic rocks) reveals their from 0.2 to 0.5 wt.% (Fig. 5). Topaz appears as an aluminous character. Vitrophyres are either slightly identifiable vapor-phase mineral in lavas whose vitro- peraluminous or metaluminous. However, topaz rhyo- phyres contain over 0.2 wt.% F. Fluorine concentrations lites are not the eruptive equivalent of S-type granites over 1 wt.% are only known from vitrophyres from Spor (e.g., White and Chappell, 1983) and are decidedly Mountain and the Honeycomb Hills complex (which different from the P-rich strongly peraluminous topaz- has magmatic topaz), both in western Utah. Compar- bearing granites that are associated with some of them isons of vitrophyre–felsite pairs almost universally (London et al., 1999; Chappell and Hine, 2006). The show that F is lost during devitrification. Chlorine is high Fe/Mg ratios of topaz rhyolites mark them as even more strongly depleted during devitrification of mostly tholeiitic (or ferroan in the sense of Frost et al., volcanic glass; no mineral phase concentrates Cl in 2001a) in contrast to the older calc-alkaline (or mag- contrast to F with its mineralogical hosts, topaz and nesian) magmatism that preceded them in most areas of fluorite. Thus, meaningful halogen concentrations can the western United States (Fig. 5). only be obtained by analysis of vitrophyres or obsidians. 230 E.H. Christiansen et al. / Lithos 97 (2007) 219–246

In such fresh rocks, Cl concentrations are typically less than 0.2 wt.% and generally much lower than this. The high F/Cl ratios (greater than about 3) of topaz rhyolites set them apart from both calc-alkaline and peralkaline rhyolites which have much lower F/Cl ratios (Chris- tiansen and Keith, 1996). Studies of melt inclusions in the Spor Mountain rhyolite show that only a small amount of the Cl and little F was lost during eruption (Zhang and Christiansen, unpublished data). Rapakivi granites, in contrast, have much lower concentrations of Cl, probably as a result of post-magmatic fluid loss. However, topaz-bearing varieties have F concentrations that commonly exceed 1 wt.% (Haapala, 1977; Rämö, 1991; Lukkari, 2002; Haapala and Lukkari, 2005). The trace element compositions of Wah Wah rhyolites show the strong enrichments of Rb, U, Th, Pb, Ga, Nb, Ta, Y, Cs, Zn, Sn, Be, and Li typical of topaz rhyolites. In contrast, Sc, Ni, Co, Cr, V, Zr, Hf, Ba, Sr, Eu, and P are all strongly depleted. P2O5 concentrations are below 0.02% in almost all topaz rhyolites, including those from the Wah Wah Mountains (Table 1). In contrast, topaz granites and pegmatites associated with strongly peraluminous magmas are phosphorous-rich (Chappell and White, 1992; Christiansen and Keith, 1996; London et al., 1999). High Ga/Al ratios of the Wah Wah rhyolites are typical with average 10,000⁎Ga/ Al of about 6. This index is used by Whalen et al. (1986) as a key indicator of A-type granites when over 2.5. REE patterns for topaz rhyolites show some variation (Christiansen et al., 1986), but they generally have low ⁎ La/CeN, La/YbN and Eu/Eu (0.45 to 0.01 for analyzed specimens). REE patterns in samples from the Wah Wah Mountains are similar to most topaz rhyolites and to Finnish topaz granites (Fig. 6). Light REE concentrations generally do not exceed 200 times chondrite values and typically they are less than about 100 times chondrite. Fig. 6. Trace element compositions of Miocene volcanic rocks from As expected, trace element patterns (Fig. 6B) show the Wah Wah Mountains compared with Proterozoic rapakivi granites strong depletions in Ba, Sr, P, and Ti, but they are most from southern Finland. (a) Chondrite-normalized REE patterns of notable for their strong enrichments of Rb, Th, and U topaz rhyolites from the Wah Wah Mountains compared with Finnish rapakivi granites (Haapala and Lukkari, 2005). (b) Primitive mantle and small negative Nb anomalies. Deep negative Nb normalized extended trace element patterns for contemporaneous anomalies are common in subduction zone rhyolites mafic and silicic lavas from the Wah Wah Mountains. Note the such as those that erupted in the Oligocene of the Great prominent negative Nb anomaly in these rift-related mafic lavas. Basin (e.g., in the Wah Wah Mountains the Oligocene Normalizing values from McDonough and Sun (1995). dacites and rhyolites of the Indian Peak volcanic field). However, negative Nb anomalies are also apparent in such as Rb, Nb, and U double in concentration. Silica the mafic magmas that accompanied the eruption of the contents are actually lower in rhyolites which are Wah Wah topaz rhyolites. extremely enriched in fluorine and incompatible elements Silica contents vary little with differentiation as seen such as the lavas at Spor Mountain and Honeycomb in individual dome complexes. For example, silica Hills, Utah. K generally declines with increasing concen- ranges from 76.2 to 77.8 wt.% in the fresh rhyolites of trations of F and other decidedly incompatible elements, the Wah Wah Mountains, whereas incompatible elements whereas Na increases. An enrichment in fluorine, Na/K, E.H. Christiansen et al. / Lithos 97 (2007) 219–246 231

and incompatible elements is also seen in the Eurajoki alkalies (average Na2O+K2O=8.4 wt.%). Typically the and Kymi topaz granites of the Finnish rapakivi com- K2O/Na2O ratios are above 1. The Fe/Mg ratios are also plexes (Haapala, 1977; Haapala and Lukkari, 2005). high, with Fe/(Fe+Mg) averaging about 0.9 (Fig. 5). Haapala (1977) and Christiansen et al. (1984) interpreted The extreme enrichments of incompatible trace elements these trends in granites and rhyolites as resulting from are exemplified by F (ranging from 0.04% to 1.53% crystallization near the minimum in the granite system with an average of 0.35%) and Rb (to over 1000 ppm). with elevated fluorine concentrations (Manning, 1981). High Ga/Al ratios are typical with 1000⁎Ga/Al varying Incompatible elements increase in concentration, from 1.8 to 15 and an average of 4.2. locally dramatically, and include Rb, U, Th, Pb, Ga, As in topaz rhyolites, differentiation trends within Nb, Ta, Y, Sn, Li, Cs, and Be. Elements that decline individual rapakivi complexes show increases in Si, F, during differentiation include Ti, Fe, Mg, Sc, Ni, Co, Cr, Ga, Rb, Sn, Nb, and decreases in Ti, Al, Fe, Mg, Mn, and V (removed by the fractionation of mafic silicates Ca, Ba, LREE, Eu, Sr, Sc, and Zr (Rämö and Haapala, and oxides), Ca, Ba, Sr, and Eu (depleted by feldspar 1995). Topaz-bearing biotite granites are usually the crystallization), as well as P, Zr, and Hf (apatite and youngest and the most highly evolved units. Compared zircon are relatively insoluble in metaluminous melts, so to less differentiated (parental?) rocks, they also have removal of these phases keeps element concentrations lower K/Na and higher Fe/Mg ratios. The highest Rb/Sr, low). Fe/Mg ratios increase with differentiation because Rb/Ba, and Ga/Al ratios are all found in the topaz- at low oxygen fugacities biotite Fe/Mg ratios are sub- bearing granites. La/YbN ratios in rapakivi granites stantially lower than Fe/Mg of melt and the fractionation average about 9, but are much lower (about 1) in the of magnetite is not pronounced. Differentiation trends topaz-bearing phases. These flat REE patterns also have for the REE show decreases in LREE and Eu and low Eu/Eu⁎ (Fig. 6). increases in HREE concentrations. The decline in LREE High Zr and Hf concentrations are the only major or probably results from the fractionation of small amounts trace element features of common rapakivi granites that of allanite, chevkinite, monazite, or other REE-rich are not characteristic of topaz rhyolites. For example, accessory phases. Likewise, in extremely evolved mag- hornblende, biotite–hornblende, and biotite granites from mas, U, Th, Y, and perhaps Nb may also decline as the Suomenniemi batholith have Zr concentrations that uraninite, thorite, xenotime, titanite, Nb-rich ilmenite, range from 600 to 400 ppm (Rämö and Haapala, 1995). and Nb oxides reach saturation and fractionate from the Zirconium concentrations are only 150 to 180 ppm in the magma (Funkhouser-Marolf, 1985). Wah Wah rhyolites (Table 1); even lower Zr concentra- In short, the elemental composition of cogenetic lavas tions are found in other topaz rhyolites. However, if the reveals the importance of crystal fractionation near the topaz-bearing phases of rapakivi granite complexes are granite minimum. Fractionation involved the removal of considered, this difference disappears. Zirconium con- sanidineNquartzNplagioclase≫biotiteNFe–Ti oxides≫ centrations range from 160 to 180 ppm in the Sääskijärvi apatiteNzirconNallanite/monazite/chevkinite. The ex- topaz granite (Lukkari, 2002), to as low as 70 ppm in the treme depletions and enrichments can be explained by Eurajoki granite (Rämö and Haapala, 1995), and range fractional crystallization of 70 to 85% of a parental from 12 to 30 ppm in some phases of the highly evolved rhyolite (Christiansen et al., 1984; Moyer and Esperança, Kymi granite (Haapala and Lukkari, 2005). Hafnium and 1988). Variable degrees of partial melting will not produce Zr are compatible elements in aluminous granitic the extreme depletions of compatible elements seen in magmas such as these; their concentrations decline with differentiated topaz rhyolites. differentiation because zircon solubility is limited at low temperatures. 7.2. Geochemistry of rapakivi granites from Fig. 7 shows the composition of topaz rhyolites and Fennoscandia Finnish rapakivi granites on several tectonic or magmatic discrimination diagrams. In each diagram, the topaz Granites in rapakivi suites have high Si, K, F, Rb, Ga, rhyolites overlap with the Finnish rapakivi granites. In Zr, Hf, Th, U, Zn, and REE (except Eu), and low Ca, general, these distinctive rhyolites and granites plot in the Mg, P, and Sr abundances compared to granitic rocks in within-plate or A-type fields as contrasted with the I-, S-, general (Rämö and Haapala, 1995). Rapakivi granites and M-type granites distinguished on the diagrams of also have high Fe/Mg, K/Na, and total alkalies. Similar Whalen et al. (1986) and volcanic arc or syn-collisional to topaz rhyolites of western United States, the Finnish granites of Pearce et al. (1984).IntheRb–Y+Nb rapakivi granites straddle the peraluminous–metalumi- diagram, topaz rhyolites from the western United States nous boundary (Fig. 5) and have high contents of and many rapakivi granites plot near the boundary 232 E.H. Christiansen et al. / Lithos 97 (2007) 219–246

Fig. 7. Topaz rhyolites plot with other metaluminous A-type silicic rocks on the discrimination diagrams of Whalen et al. (1986) and Pearce et al. (1984). Rapakivi granites, especially those with topaz, plot in similar positions except on the Nb–Y diagram where most topaz rhyolites are richer in Nb and/or poorer in Y. Subdivision of within-plate granite field in (d) from Eby (1992). between WPG (within-plate granites) and syn-COLG Grove (Fig. 2). The hydrothermal alteration at Pine (syn-collisional granites) (Fig. 7). On the Nb–Ydiagram Grove includes topaz. Younger topaz rhyolite dikes cut many topaz rhyolites are richer in Nb and/or poorer in Y the buried intrusion, but the relationship to the erupted than most rapakivi granites. However, here again, most of topaz rhyolites is unclear. The Mo-mineralized magma the topaz granites have lower Y/Nb ratios than parental system did erupt a garnet-bearing rhyolite tuff (Keith rapakivi granites. Fractionation of xenotime or titanite et al., 1986). Elsewhere in the western United States, may raise the partition coefficients for Y and thereby large deposits of Be (as bertrandite) and Climax-type decrease Y/Nb to lower values during differentiation. Mo(W) deposits, small deposits of U and F, and sub- economic occurrences of red beryl, Li, Cs, and Sn are 8. Metallogeny directly related to topaz rhyolites. The rhyolites are contemporaneous with the deposits, co-magmatic with 8.1. Metallogeny of topaz rhyolites mineralized intrusions, and, in some cases, hosts of the ore. The marked magmatic enrichment of these same Mineral deposits associated with topaz rhyolites in elements strongly suggests that the ore elements were the Wah Wah Mountains include small deposits of derived from the rhyolites (or their intrusive forerunners fluorite, uranium, alunite, and native S, as well as gem in the case of Climax-type Mo deposits; Burt et al., red beryl in one of the older flows (Christiansen et al., 1982). Other types of mineralization (alunite, S, Hg, 1996; Thompson, 2002). A large, but unexploited, Au–Ag) are spatially and temporally associated with Climax-type porphyry Mo deposit is located at Pine some topaz rhyolites (e.g., John, 2001). The association E.H. Christiansen et al. / Lithos 97 (2007) 219–246 233

9. Isotopic compositions

9.1. Isotopic compositions of topaz rhyolites from the Wah Wah Mountains

Initial Sr, Nd, and Pb isotope ratios for topaz rhyolites from the Wah Wah Mountains are reported in Table 2. The Pb isotope ratios of the silicic rocks are indistinguishable from those of the contemporaneous mafic lavas, except for the 208Pb/204Pb ratios which suggest that the silicic magmas were derived from sources with slightly higher Th/Pb ratios than the sources of the mafic magmas. Considerable uncertainties exist in the initial 87Sr/86Sr ratios because of the extremely high Rb/Sr ratios of the rocks. The error bars Fig. 8. SiO2 versus Zn for volcanic rocks from the Wah Wah Moun- tains of southern Utah. Topaz rhyolites (and other young bimodal show the effects of recalculation by assuming a 1 ppm rhyolites) are much richer in Zn than calc-alkaline subduction-related difference in the Sr concentration of these low Sr rocks. silicic rocks of Oligocene age that erupted about 10 m.y. earlier. Our best estimate is that initial 87Sr/86Sr ratios for topaz rhyolites from the Wah Wah Mountains are between 0.706 and 0.710. A further constraint is that the initial of these latter deposits with the rhyolites may rely more 87Sr/86Sr ratio of the topaz-bearing Sheeprock granite on magmatic heat content and volcanologic style for from western Utah is 0.7064 as taken from a Rb–Sr their generation than on any particular compositional isochron (Christiansen et al., 1988). Care must be taken feature of topaz rhyolites. because a small amount of upper crustal contamination would significantly raise the Sr isotope ratios of these 8.2. Metallogeny of rapakivi granites low Sr magmas. For example, Reece et al. (1990) modeled an increase of the initial 87Sr/86Sr ratio from Greisen-type Sn–Be–W–Zn mineralization (with 0.705 to 0.712 as resulting from the assimilation of only beryl, genthelvite, and bertrandite as the Be minerals) 1% of radiogenic upper crustal wall rocks into Taylor is associated with the topaz-bearing granites of southern Creek Rhyolite of New Mexico's Black Range. Finland (Haapala, 1977; Edén, 1991; Haapala, 1995, These low initial 87Sr/86Sr ratios and the implied low 1997). This element association is similar to that of Rb/Sr ratios of the sources were originally attributed to topaz rhyolites, with the possible exception of Zn. But it derivation of topaz rhyolites from felsic granulitic rocks in should be noted that many bimodal rhyolites (as well as the lower continental crust with little or no involvement of rapakivi granite suites) are exceptionally rich in Zn for mantle or “juvenile” crust (Christiansen et al., 1983, their high SiO2 contents (Fig. 8). 1988). However, new Nd isotopic data from the Wah Wah

Table 2 Strontium, neodymium, and lead isotopic compositions of Miocene lavas from the Wah Wah Mountains, Utah 87 86 206 204 207 204 208 204 Sample # Rock Type Sr/ Sr (T) eNd (T) Pb/ Pb Pb/ Pb Pb/ Pb Steamboat Formation (13 to 11 Ma) BBS-8-229-2 Topaz rhyolite 0.710⁎ −11.4 18.1 15.6 39.4 LAM-9-103-2 Topaz rhyolite 0.710⁎ −9.7 18.2 15.6 39.3 LAM-9-103-3 Bas trachyandesite 0.7054 −7.2 17.5 15.5 37.6 LAM-956-1 Trachyandesite 0.7052 −6.8 18.1 15.6 38.1

Blawn Formation (22 to 18 Ma) LAM-1-38-2 Topaz rhyolite 0.710⁎ −9.3 18.2 15.6 39.4 TET-9-43-2 Topaz rhyolite 0.710⁎ −9.5 18.1 15.6 39.3 BAN-8-127-2 Bas trachyandesite 0.7065 −4.4 18.7 15.6 38.9 FRSC-2-62-3 Trachyandesite 0.7060 −8.8 18.3 15.6 38.6 Procedures and errors are described in the text. ⁎ 87Sr/86Sr (T) for topaz rhyolites have uncertainties on the order of +/−0.005 because of extremely high Rb/Sr ratios. 234 E.H. Christiansen et al. / Lithos 97 (2007) 219–246

Mountains are inconsistent with that interpretation (Table 2 and Fig. 9). The Nd isotope ratios are higher (εNd −9to−11) in the topaz rhyolites from the Wah Wah Mountains than in the slightly older calc-alkaline suite (εNd −12 to −19) and other granites and rhyolites from the eastern Great Basin, but the Nd isotope ratios are slightly lower than in the contemporaneous mafic magmas (εNd −4to−9). Also plotted on Fig. 9a, are the compositions of average mafic and felsic xenoliths from the Colorado Plateau (Condie et al., 1999), which may be the best current estimates of the lower crustal composition in this part of the western United States. Although it is clear that the topaz rhyolites of the Wah Wah Mountains cannot be derived simply by partial melting of a felsic component in the crust, they could be derived from partial melts of a mixture of felsic and mafic crustal components of Proterozoic age. Alternatively, the Sr and Nd isotopic data could be explained by the inclusion of an important mantle component or “juvenile” crustal component composed of young mantle-derived mafic magmas (represented by the contemporaneous mafic volcanic rocks) trapped in the crust and then re-melted.

9.2. Isotopic composition of rapakivi granites

Rämö and Haapala (1995) and Haapala et al. (2005) have reviewed the isotopic compositions of many rapakivi granites. As noted earlier, most rapakivi granites are only a few hundred million years younger than the Proterozoic crust into which they intruded. Thus, it is difficult to clearly ascertain the nature of their sources using slowly changing Nd isotope ratios. For example, rapakivi granites from southern Finland (εNd 0to−3) lie within the broad isotopic evolution field for 1.9 Ga Svecofennian crust, but because the granites are only 300 million years younger than this crust, their derivation entirely from continental crust is not assured. The εNd values (+1.6 to −1.7) of rapakivi-age diabase Fig. 9. Isotopic composition of volcanic rocks from the Indian Peak volcanic field of the Wah Wah Mountains, southern Utah. (a) Topaz dikes and other mafic rocks largely overlap those of the rhyolites (black circles) are very distinct from the felsic xenoliths from rapakivi granites (Rämö, 1991; Haapala et al., 2005). the lower and middle crust (Ave felsic crust) of the Colorado Plateau Rapakivi granites that intrude older Archean terranes are analyzed by Condie et al. (1999). If topaz rhyolites are derived from rare, but the isotopic evidence for the nature of their ancient crust, then the crust must be more similar to mafic xenoliths sources is clearer. For example, rapakivi granites and (Ave mafic crust) of the Colorado Plateau examined by Wendlandt et al. (1993) which have much higher Sm/Nd ratios than typical gabbro-anorthosites of the Salmi batholith in Russian Proterozoic crust. Alternatively, young mantle-derived magmas may Karelia (εNd at 1540 Ma −6.2 to −9; Rämö, 1991; have been a significant component in the sources of topaz rhyolites Neymark et al., 1994) and the rapakivi granites of the from the western United States. (b) Miocene topaz rhyolites from the Shachang complex in northeastern China (εNd at Wah Wah Mountains have distinctly higher epsilon Nd values than 1685 Ma about −5.7; Rämö et al., 1995; Haapala Oligocene dacite and rhyolite (gray circles) from the region and plot ε far above the evolution curve for Proterozoic crust (Miller and et al., 2005) have much higher Nd values (5 to 7 Wooden, 1994). epsilon units) than the Archean crust they intrude or than crustal evolution curves with typical Sm/Nd ratios for E.H. Christiansen et al. / Lithos 97 (2007) 219–246 235 felsic crust. This could be the result of mixed crustal rhyolites may not provide a test for the origin of the sources consisting of Archean and Proterozoic compo- rapakivi texture. nents or it could mean that juvenile mantle-derived components were involved in their generation. In 10.3. Bimodality addition, isotopic evidence shows that parts of the Sherman batholith of Wyoming cannot be derived solely Topaz rhyolites are mostly, but not exclusively, from the felsic Archean crust it intrudes (Frost et al., members of bimodal volcanic series. This is apparent 1999, 2001b). The Cretaceous bimodal A-type granite for the Wah Wah Mountains in Fig. 5 and for the northern complexes of western Namibia (including the Spitz- Basin and Range Province as a whole in Fig. 10.Before koppe topaz-bearing granites described by Frindt et al., about 25 Ma, volcanism was dominated by a nearly 2004b) that are related to continental rifting and mantle- continuous series of andesite, dacite, and rhyolite compo- plume activity show geochemical and isotopic (Nd, Sr, sitions. After this date, intermediate magmatism declined O) compositions that suggest varying mixing relations in importance and basaltic magmas erupted. Topaz between plume-related mantle magmas and lower rhyolites formed during this younger, bimodal volcanism. crustal rocks (Trumbull et al., 2004; Frindt et al., Likewise, rapakivi granites are often associated with 2004a). The initial 87Sr/86Sr ratios and εNd values of the mafic rocks ranging from gabbros to anorthosites, topaz-bearing granite are distinct from the metasedi- ferrodiorites, and basaltic dikes (Rämö and Haapala, mentary gneisses it intrudes (McDermott and Hawkes- 1995). The common bimodality of these distinctive A- worth, 1990; Seth et al., 1998) but overlap substantially type magmas must be accounted for in any successful with one type of the contemporaneous Etendeka flood petrogenetic model. basalts. εNd ranges from −4to−7 for the LTZ.L flows, which are interpreted to have interacted extensively with 10.4. Tectonic setting: orogenic, anorogenic, mafic lower crust (Ewart et al., 1998a,b). or taphrogenic

10. Discussion None of the topaz rhyolites of the western United States were erupted during periods of contractional 10.1. Topaz rhyolites are the eruptive equivalents of rapakivi granites

Based on similarities in the mineralogy, calculated volatile fugacities, elemental and isotopic compositions, and metallogeny, we conclude that topaz rhyolites of the western United States are the Cenozoic equivalents of highly evolved rapakivi granite magmas. As such, they may help us better understand the origin and evolution of rapakivi granites and other A-type felsic magmas. In the following paragraphs, we try to synthesize the most important interpretations that derive from this conclusion.

10.2. Textures

Fig. 10. SiO2 (normalized on anhydrous basis) content versus time for Rapakivi textures have not been found in any topaz igneous rocks of the eastern Great Basin (east of 87Sr/86Sr line in Fig. 1). rhyolite from the western United States, in spite of other Middle Cenozoic magmatism was dominated by intermediate to silicic evidence for magma mixing, rapid decompression that magmatism. Beginning between 25 and 20 Ma mafic lavas were erupted together with high-silica rhyolites and intermediate compositions were accompanied eruption, and other changes in the physical rare. Topaz rhyolites erupted as an important part of this late Cenozoic and chemical conditions that have been suggested as bimodal volcanism. Data from middle Cenozoic compilation of Barr causing the distinctive rapakivi texture (Rämö and Haa- (1993), augmented by data from Vitaliano and Vitaliano (1972), Clark pala, 1995; Eklund and Shebanov, 1999). The rapakivi (1977), Best et al. (1980), Ekren et al. (1980), Novak (1984), texture is also absent in the topaz-bearing phases of Christiansen et al. (1986), Fitton et al. (1988), Coleman and Walker (1992), Moore (1993), Miller and Wrucke (1995), Brooks et al. (1995), rapakivi complexes of southern Finland. Instead, the Rogers et al. (1995), Fleck et al. (1996), Beard and Johnson (1997), texture is most common in the more mafic hornblende- Nelson and Tingey (1997), Cunningham et al. (1998), Smith et al. bearing phases of the composite intrusions. Thus, topaz (1999), DePaolo and Daley (2000),andClark (2003). 236 E.H. Christiansen et al. / Lithos 97 (2007) 219–246 orogenesis; almost all are clearly related to extension truly anorogenic setting. For example, laccoliths in (taphrogeny). A few of the older rhyolites may have southern Utah and Montana are about the same age as erupted during a nearly neutral tectonic environment some of the older topaz rhyolites in these regions. Fig. 11 with the least principal stress oriented vertically—a outlines our interpretation of the tectonic evolution of the western United States (e.g., Best and Christiansen, 1991). Before about 45 Ma, shallow subduction of oceanic lithosphere beneath the western United States shut off magmatism over a broad area and caused widespread folding, thrust faulting, and crustal thickening during the Sevier and Laramide orogenies. Between 45 Ma and about 22 Ma (Fig. 11b), the shallow oceanic slab dropped away from the lithosphere (slab rollback). The immersion of the slab into the hotter asthenosphere caused it to dehydrate over a broad area and induced the formation of magmas with subduction zone signatures far from the coastal trench. The rollback of the slab appears to have started beneath Montana and progressed southward as an east-trending bend or as a series of tears in the plate propagated in that direction. This is evidenced by the southward moving front of calc-alkaline magmatism that swept across the western United States during this time period. These magmas are calc-alkaline andesites to rhyolites with strong subduction-related geochemical signatures. Composite volcanoes and caldera complexes were the principal volcanic features. After about 22 Ma (Fig. 11c), counterflow of hot asthenosphere to replace the cold lithosphere appears to have dominated the tectonism in the region. It was accompanied by extension and lithospheric thinning below what is now the Basin and Range province. Extension and decompression were accompanied by enhanced heat flow from the counterflow of the asthenosphere. As a result, partial melting created basaltic magmas that erupted or were lodged in the base of the crust. Rhyolites, many of which bear topaz, formed and erupted concurrently. Slab rollback followed almost immediately by extension may help explain the conflicting evidence for nearly simultaneous formation of extension- and subduction-related igneous rocks.

Fig. 11. Cenozoic tectonic evolution of the western United States is shown in these schematic cross sections. (a) Before 45 Ma, shallow subduction of oceanic lithosphere beneath the western United States shut off magmatism over a broad area and caused contractional deformation. (b) Between 45 and about 22 Ma, the shallow slab dropped away from the lithosphere creating a southward moving front of calc-alkaline magmatism. (c) After about 22 Ma, counterflow of hot asthenosphere and lithospheric extension caused the lithosphere to thin. Decompression melting and enhanced heat flow created within- plate basaltic magmas that erupted or intruded the crust becoming hybridized in the process. (d) Partial melting of the hybridized lower crust created rhyolitic magma which differentiated and assimilated crust to become topaz rhyolites. E.H. Christiansen et al. / Lithos 97 (2007) 219–246 237

Recent studies of the tectonic setting of the Fennos- are about 20 km across. Distinctive fractional crystal- candian rapakivi granites show several similarities to lization trends have been identified in many of these that of the Cenozoic rhyolites of the western United composite flow fields. Minor contamination by wall States. For example, there is usually no evidence of rocks has also been identified in the Black Range (Reece concurrent compressional orogenic movement in the et al., 1990). fabric of Finnish rapakivi granites (Rämö and Haapala, Clearly there are great difficulties in comparing the 1995). Instead, they are contemporaneous with silicic volumes of intrusive and extrusive rocks. However, and mafic dikes suggesting concurrent extension. some important relationships are apparent. Finnish Geophysical studies show that the crust, and especially rapakivi granites are exposed in four large and eleven the lower crust, is markedly thinner beneath the rapakivi smaller composite magmatic complexes. Outcrop areas granites. Proterozoic graben structures and faults, of the composite batholiths range from almost suggesting intracontinental rifting, have been identified 20,000 km2 (the Wiborg massif) to less than 10 km2 by seismic reflection data (Korja et al., 1993), and there for the smallest complexes. The topaz-bearing intrusions is new evidence that after emplacement of the rapakivi (Eurajoki, Kymi, Ahvenisto, Suomenniemi, Artjärvi and granites and related rocks, thick fluvial sediments Sääskärvi) are at most a few kilometers across. The small started to fill developing rift zones (Kohonen and sizes of these intrusions compared with the much larger Rämö, 2005). Haapala et al. (2005) concluded that the hornblende–biotite– and biotite–granite intrusions are mafic and silicic magmas were produced during consistent with the geochemical evidence for extensive incipient rifting of the Proterozoic continental crust. fractional crystallization of silicic magma to form highly Apparently, extension was important in the setting in F-rich residual magmas. On the other hand, the topaz which rapakivi granites formed and local upwelling of granites have more variable εNd values than surrounding the mantle may also have been important. There is no “normal” rapakivi granites (Rämö, 1991), which points evidence for contemporaneous subduction-related oro- to possible differences in their sources. In any case, the genesis in Poland or the Baltic states to the south of the close association of topaz granites and topaz rhyolites in rapakivi batholiths. However, in southwestern Sweden the apical parts of the epizonal rapakivi complexes in and southern Norway, 500 to 1500 km to the southwest Finland may imply that the topaz rhyolites of western of the rapakivi batholiths, there are 1.69 to 1.50 Ga calc- United States are underlain by much larger composite alkaline plutonic and volcanic rocks that represent a granitic batholiths that have not been exposed by uplift subduction-related magmatic arc at the margin of the and erosion. Fennoscandian shield (Åhäll et al., 2000; Andersen et al., 2004). Subduction-related mantle flow at the 10.6. Halogen concentrations margin of the continent may have contributed to the mantle upwelling in the inner parts of the thickened Both fluorine and chlorine are incompatible ele- continent (e.g., Hoffman, 1989; Åhäll et al., 2000; ments; Cl is probably more incompatible than fluorine. Haapala et al., 2005). Thus, a taphrogenic origin far Thus, because A-type silicic magmas are enriched in all behind a contemporaneous magmatic arc seems likely incompatible elements, it is expected that halogen for the Svecofennian rapakivi granites. concentrations also will be high. Generally, this is the case with F; concentrations in glassy topaz rhyolites are 10.5. Volumes of magma and the role of fractional 3 to 15 times higher than in average glassy calc-alkaline crystallization rhyolites (e.g., Macdonald et al., 1992). However, F is much more enriched than Cl imparting the high F/Cl Topaz-bearing phases of rapakivi batholiths are ratios found in these rock suites (Fig. 5). The origin of usually late and small (at most a few kilometers across), the high F/Cl ratios is still problematic. Hypotheses but most if not all, are parts of larger complexes from include: 1) high F/Cl ratios are related to a granulitic which the topaz granites probably evolved by fractional crustal source that had high F/Cl ratios in residual biotite crystallization (e.g., Haapala, 1997). For topaz rhyolites, that broke down during fluid-absent melting (Collins estimates of eruption volumes range from a few tenths et al., 1982; Christiansen et al., 1983), 2) high F/Cl ratios of a cubic kilometer for small domes less than 1 km are not primary characteristics of the magmas, but instead across to as much as 50 km3 for the composite lava result from separation of a Cl-rich magmatic fluid, fields in the Thomas Range of Utah (Christiansen et al., perhaps even during eruption (Christiansen et al., 1986; 1986) and the Black Range of New Mexico (Duffield Webster, 1992), 3) high halogen concentrations and high and Dalrymple, 1990). The large composite flow fields F/Cl ratios are the result of fractional crystallization of 238 E.H. Christiansen et al. / Lithos 97 (2007) 219–246 mantle-derived magmas that were unrelated to subduction crystallization of mafic magma trapped in the lower and had high F/Cl ratios, or 4) high F/Cl ratios are caused crust would produce a solidified rock with relatively by partial melting of underplated mafic rocks recently high F/Cl ratios because F can be incorporated in some derived from the mantle. silicates and phosphates in preference to Cl which is As noted by many others, granulites have small probably lost in a hydrothermal fluid. Subsequent partial amounts of hydrous phases–either amphibole or biotite– melting of this mafic plutonic rock would yield a mag- that are enriched in F compared to Cl and water because ma with a high F/Cl ratio but low overall halogen of the higher thermal stability of F-rich end members. concentrations. Decomposition of the small amount of hydrous minerals We prefer the last explanation for the high F/Cl ratios could produce a small amount of F-rich, high F/Cl ratio by a process of elimination of other hypotheses and silicic melt. However, the isotopic and trace element data because it agrees with the isotopic evidence for a do not support the generation of topaz rhyolites solely significant “mantle” component in the topaz rhyolites. from typical crustal materials, including felsic granulites This proposal does not place much of a constraint on the derived by metamorphism of calc-alkaline igneous rocks ultimate origin of the source rock, however, because even with or without a sedimentary component. Thus, al- calc-alkaline subduction zone magmas (which have low though attractive at first, we have rejected this hypothesis F/Cl ratios) crystallize to form rocks with high F/Cl ratios. as untenable. For example Christiansen and Lee (1986) concluded that Studies of melt inclusions in topaz rhyolites are in the average F/Cl ratio of granitic rocks from the Basin and their infancy, but thus far show that melt inclusions in Range province is 3.3, but F and Cl concentrations are low quartz have high F/Cl ratios similar to the obsidians and (e.g., 0.043 and 0.013 wt.% respectively). vitrophyres (unpublished studies by X. Zhang and E.H. Christiansen). This suggests that the high F/Cl ratios are 10.7. Sources and partial melting history not simply the result of devolatilization during eruption and preferential loss of Cl into the fluid or vapor. High Several sources for the generation of A-type gra- F/Cl ratios have been found in studies of “melt” inclu- nites have been proposed, including: 1. anatexis of sions in the topaz granites of Finland as well (Haapala felsic calc-alkaline (I-type) crust, 2. partial melting of and Thomas, 2000). felsic granulites depleted in incompatible trace elements Few potentially parental mafic magmas have the by earlier high-grade metamorphism or melt extraction, requisite high F/Cl ratios to be direct parents of alumi- 3. partial melting of distinctive ancient mafic lower crust, nous A-type granites (Fig. 5). For example, arc magmas 4. fractional crystallization of mantle-derived magma have characteristically low F/Cl ratios (0.2 to 1; e.g., (with or without assimilation of felsic crust), and 5. partial Bacon et al., 1992; Christiansen and Keith, 1996)asa melting of juvenile mafic or hybridized intermediate crust result of the enrichment of Cl introduced into the mantle created by intrusion of basaltic magma. We consider each wedge by dehydration of the subducting slab of oceanic below. lithosphere. The much higher solubility of Cl than F in hydrothermal fluids (Webster, 1992) enriches the fluid 10.7.1. Anatexis of felsic calc-alkaline crust and then the resulting partial melt in Cl. Closed system Patino-Douce (1996) concluded, on the basis of par- differentiation of these magmas produces silicic magmas tial melting experiments with a granodiorite and a and eventually ore deposits with low F/Cl ratios unlike tonalite from the Sierra Nevada batholith, that A-type topaz rhyolites. granites could form by low pressure (∼4 kb) partial Glassy rinds and melt inclusions of midocean ridge melting of normal (= felsic calc-alkaline) continental basalts do have high F/Cl ratios (Byers et al., 1986), but crust. He based this conclusion on a comparison of the it is very unlikely that any of the other elemental or major element characteristics of A-type granites with the isotopic characteristics of A-type magmas could be the partial melting experiments. Important discriminating result of differentiation of MOR basalt. Glassy, plume- factors were modest K+Na/Al, high Fe/Mg, and high related rocks have moderate F/Cl ratios (≤2) and their Ti/Mg ratios in the low pressure experiments. However, differentiation produces magmas with similar ratios, but the high Fe/Mg and Ti/Mg ratios in the experimental high halogen concentrations. These are the character- liquids were the result of partial melting at a very low istics of peralkaline A-type magmas which are com- f O2 imparted by using a graphite-based cell assembly. monly thought to be derived by fractionation of mildly Estimated f O2 of the experiments was 1 log unit below alkalic mafic magmas (Mahood and Baker, 1986; the QFM oxygen buffer. Apparently, the low oxygen Scaillet and Macdonald, 2001). On the other hand, fugacity and low pressure restricted the crystallization of E.H. Christiansen et al. / Lithos 97 (2007) 219–246 239 titaniferous magnetite (ilmenite and rutile are the only Another important problem with any model involv- Fe–Ti oxides noted) and elevated Fe and Ti in the melt. ing melting of felsic crust is the common association of Abundant Fe2+ made the residual pyroxenes Fe-rich. In A-type granites with peralkaline magmas. Peralkaline contrast, normal calc-alkaline granodiorites and tona- silicic rocks are not only the same age as some topaz lites are strongly oxidized having initially crystallized at rhyolites and rapakivi granites, but they also occur oxygen fugacities as much as 104 to 105 times higher within the same magmatic complexes. Small volumes of than the experimentally imposed buffer. Moreover, as topaz-bearing lava erupted in the middle of the Kane argued by Carmichael (1991), the oxygen fugacity of a Springs Wash caldera, Nevada (Novak, 1984), and magma or its source is particularly resistant to change. peralkaline syenite dikes are included in the Suomen- Thus, it is unlikely that partial melting of normal crustal niemi complex of southern Finland (Rämö, 1991). granodiorites would occur at such a low f O2. Skjerlie Generating peralkaline magmas by melting metalumi- and Johnston (1992) also were able to produce F-rich nous source rocks typical of the continental crust would granitic melts with high Fe/Mg ratios from partial require unusual circumstances. melting of tonalite. However, their experiments were also done at low oxygen fugacities near the QFM buffer. 10.7.2. Partial melting of felsic granulites A related problem with melting calc-alkaline crust to A lower crustal felsic granulite is a more likely get A-type magmas, including topaz rhyolites and rapa- magma source because depletion of incompatible and/or kivi granites, involves the generation of the low f O2 soluble lithophile elements (like Rb) could erase or found in the reduced varieties (e.g., Frost and Frost, decrease the Rb/Nb ratio, produce low Rb/Sr ratios, and 1997). Unless it includes large volumes of reduced sedi- water-poor, but F-rich rocks. Inherently small degrees of mentary carbon, continental crust composed of normal melting could then give rise to incompatible element- calc-alkaline igneous rocks is oxidized (2 or more log rich rocks. Thus, the derived magma would have a small units above the QFM oxygen buffer, e.g., Carmichael, Nb anomaly and a low Sr isotope ratio—important 1991) because of its generation in subduction zones. characteristics of topaz rhyolites and rapakivi granites Partial melting of such oxidized crust also gives oxidized (e.g.,Christiansen et al., 1988). However, felsic granu- magmas, not the reduced tholeiitic (or ferroan types) so lites have Sm/Nd ratios that are similar to other crustal common among A-type magmatic suites. rocks, and thus develop low εNd values over time Moreover, partial melting of a typical calc-alkaline (Rudnick and Fountain, 1995; Condie et al., 1999). As granodiorites or diorites will not produce the trace ele- noted above, the relatively high Nd isotope ratios of ment characteristics of A-type granites, such as those Cenozoic topaz rhyolites are inconsistent with deriva- discussed here. For example, Rb/Nb ratios in calc- tion solely from ancient (i.e., Proterozoic) felsic crust. alkaline (I-type) granitic rocks (apparent in volcanic arc, Contrasting opinions prevail for some Precambrian subduction zone, and in average continental crust) are A-type granites. For example, DePaolo (1981) and rather high and not easily changed by partial melting Bennett and DePaolo (1987) used Nd isotopic evidence since both Rb and Nb are incompatible elements. Nega- to conclude that the Proterozoic anorogenic granites of tive Nb anomalies such as those found in typical con- the western United States were derived by melting of tinental crust are hard to erase by partial melting pre-existing crust. However, as pointed out by Johnson processes (Christiansen and Keith, 1996). In fact, Nb (1993), basaltic rocks derived from the lithospheric is probably slightly more compatible than K, Rb, U, mantle can have εNd values as low as −11‰. If such and Th during dehydration melting involving biotite and mafic rocks are part of the source of the Proterozoic Rb/Nb ratios may increase during crustal melting (e.g., anorogenic granites of the southwestern United States, Frindt et al., 2004a,b). then the “mantle” component could be as high as 60%. Nd and Sr isotopic compositions of topaz rhyolites and A-type granites in general are difficult to explain by 10.7.3. Partial melting of mafic lower crust partially melting calc-alkaline igneous rocks in the crust. Topaz rhyolites of the western United States could be Typical Rb/Sr and Sm/Nd ratios and the Precambrian age the result of partial melting of a distinctive lower crustal of many of the country rocks and underlying lithosphere reservoir of Proterozoic age not typically “sampled” by create higher 87Sr/86Sr and lower 143Nd/144Nd ratios than rising magmas in orogenic settings. To explain the are found in these rocks. As noted above, even young relatively high εNd values of topaz rhyolites, this lower middle Cenozoic calc-alkaline dacites, rhyolites, and crustal source would need to have a high Sm/Nd ratio granites have higher 87Sr/86Sr and lower 143Nd/144Nd similar to some of the mafic xenoliths transported to the than found in the topaz rhyolites. surface of the Colorado Plateau and in the transition 240 E.H. Christiansen et al. / Lithos 97 (2007) 219–246 zone with the Basin and Range province (Esperança over a mantle plume would produce the same type of et al., 1988; Wendlandt et al., 1993; Chen and Arculus, magmatic evolution.) This could explain their high εNd 1995). For example, Condie and Selverstone (1999) values and strong enrichment in incompatible elements. speculate that the lower crust of the Colorado Plateau is For example, McCurry et al. (in press) concluded that composed mostly of amphibolite and mafic granulite small volumes of extremely evolved A-type rhyolites on with an additional 25% tonalite or diorite. These mafic the Snake River Plain were produced by fractional xenoliths have an average εNd of −3.5 and range from 7 crystallization of basaltic parents through ferrolatitic to −13 (Esperança et al., 1988; Wendlandt et al., 1993), compositions. The principal problem with this hypoth- in spite of their Proterozoic ages (Fig. 8). Important esis is the lack of intermediate composition magmas in questions that need to be answered about this potential topaz rhyolite associations and in many, but not all, source include: Would silicic partial melts have the rapakivi granite complexes. Perhaps the dense, Fe-rich requisite trace element characteristics—especially high intermediate composition magmas are too dense to be incompatible element concentrations, low Rb/Nb ratios, shallowly emplaced and are trapped in the middle crust and high F? All of the mafic xenoliths from the Colorado (e.g., Christiansen and McCurry, in press). Plateau analyzed by Mattie et al. (1997) have negative Nb anomalies and high Rb/Nb ratios. Could mafic lower 10.7.5. Partial melting of juvenile or hybridized mafic crust melt to produce low f O2 magmas with high Fe/Mg crust and F/Cl ratios? Most of the xenoliths examined so far Finally, topaz rhyolites (and by extension rapakivi have low Fe/Mg ratios like other magnesian (or calc- granites) could be partial melts of mafic intrusive systems alkaline in the classification of Miyashiro, 1974)rocks within the crust (Fig. 11d). Coeval mafic magma may and would probably melt to produce magmas with have lodged in the crust as dikes and sills and become relatively high f O2. These questions can only be an- variably hybridized by interaction with older, more felsic swered by further mineralogical investigations of mafic crustal rocks. Re-melting could have been caused by lower crustal xenoliths from the region. subsequent intrusions of hot mafic magma and heat from Predominantly mafic lower crustal xenoliths with the rising asthenosphere (e.g., Frost and Frost, 1997; ages close to the Archean–Proterozoic boundary have Streck, 2002; Christiansen and McCurry, in press). been found in kimberlites in eastern part of the Strong fractional crystallization of this partial melt– Fennoscandian shield. Mafic xenoliths from southern accompanied by more assimilation of older continental Kola peninsula show episodes of magmatic growth crust–could then produce the highly evolved topaz (2.5–2.4 Ga) and reworking (1.7 Ga) and have widely rhyolites and granites. Many late Cenozoic basalts from varying Nd isotopic compositions (εNd values at the western United States lack negative Nb anomalies 1.54 Ga commonly between −5 and −9), overlapping unless contaminated by continental crust (Lum et al., the compositions of both the rapakivi granites and the 1988; Moyer and Esperança, 1988; Barr, 1993; Smith associated gabbroic rocks of the Salmi batholith (Ney- et al., 1999). Such a young gabbroic source would ex- mark et al., 1994; Kempton et al., 2001). Mafic lower plain the high εNd, the overlap of the Pb isotopic crustal xenoliths from the Kuopio area in eastern composition of the mafic and silicic rocks, the typically Finland imply episodes of major growth (2.7 Ga) and high Fe/Mg and F/Cl ratios, the low f O2,lowRb/Nb reworking (including K-metasomatism at 1.8 Ga) and ratios and lack of large negative Nb anomalies, the have εNd (at 1.64 Ga) values (−3.5, −2.5, and +2.8) association with mafic magma in bimodal volcanic fields, grossly matching the Wiborg rapakivi granites (see Hölttä and the association with peralkaline magmas, which may et al., 2000; Peltonen and Mänttäri, 2001). However, be derived by fractionation of the mafic end member or partial melting models of hornblende-rich xenoliths and partial melting of alkali basalt lodged in the lower crust. plagioclase–clinopyroxene-rich garnet-bearing xenoliths The most significant problem with this hypothesis from the Kuopio area (Elliott, 2003) do not support the may be the large volumes of some of the composite interpretation that rapakivi granites are derived by simple rapakivi batholiths. Partial melting experiments (Helz, partial melting of these mafic rocks. 1976; Spulber and Rutherford, 1983)andMELTS models (Ghiorso and Sack, 1995) show that 10 to 10.7.4. Fractional crystallization of mantle-derived 30% melting of gabbro or ferrodiorite can yield felsic magma magma with about 65% to 73% silica, which could then Topaz rhyolites and rapakivi granites could be the differentiate to high-silica rhyolite. However, if the result of fractional crystallization of basaltic magma rapakivi batholiths are 5 to 10 km thick, as noted above, formed during continental rifting. (Elsewhere passage and if they were produced by 10% partial melting, then E.H. Christiansen et al. / Lithos 97 (2007) 219–246 241 melt must have been extracted from a melting interval in crust with its orogenic geochemistry is in general not the the crust that is 50 to 100 km thick. Greater degrees of dominant source of topaz rhyolites or rapakivi granites. partial melting would, of course, lower the thickness of Instead, we maintain that the sources of these distinc- the melting interval, as would focusing of melt formed tive A-type silicic magmas must include a significant over a larger area into a small region. Additionally, greater mantle-derived component of within-plate character. volumes of silicic magma could have been produced if the Some A-type rhyolites could be formed by extreme intrusive complex was intermediate in composition as the fractional crystallization of mantle-derived basaltic result of hybridization with felsic crust. magma. If so, a crustal density filter may serve to suppress the eruption or shallow emplacement of dense, 11. Conclusions Fe-rich intermediate composition magmas creating the characteristic bimodal associations. In most cases, The Cenozoic topaz rhyolites from the western however, this mantle component probably comes from United States and rapakivi granites from southern partial melting of coeval more or less hybridized, Finland are very similar. The rhyolites are especially gabbro–diorite intrusive complexes to produce rhyolitic similar to the late, highly differentiated, topaz-bearing (rapakivi granite) magma that fractionates to highly phases of the intrusive complexes. Similarities include evolved topaz rhyolite (topaz granite). The isotopic, elemental and isotopic compositions, mineral assem- chemical, and bimodal character of many examples of blages, mineral compositions, volatile fugacities, metal- this type of magmatism would thus be explained. In logenic associations, and inferred tectonic settings. We either case, subsequent assimilation of middle or upper conclude that they must have formed and differentiated crustal rocks may further mask the mantle signature by similar processes, even though separated by creating magmas with intermediate isotopic composi- thousands of kilometers and billions of years. Conse- tions (in Sr, Nd, and O) and the small negative Nb quently, the rhyolites may shed light on the origin of anomalies. rapakivi granites and A-type granites in general. On the other hand, many investigators of aluminous Likewise, the granitic rocks should tell us about the A-type magmas conclude that they are solely derived by nature of batholiths related to topaz rhyolites. partial melting of older continental crust with the heat We find that modern topaz rhyolites and A-type derived from underplating of mafic mantle-derived rhyolites are common in extensional (taphrogenic) magma (e.g., Creaser et al., 1991; Haapala and Rämö, settings. Young A-type rhyolites and granites are also 1992; Patino-Douce, 1996; Frindt et al., 2004a; found above mantle plumes. Together, we consider Anderson and Morrison, 2005). these global tectonic environments to be anorogenic in Important tests of these contrasting conclusions will that they contrast with the orogenic settings where most be centered on comparisons of A-type magmas through granitic magmas are formed. The link between tectonic geologic time. Future studies should focus on isotopic, setting and A-type magma characteristics seems to lie trace element, and mineralogic studies of the composi- with the mantle-derived magmas found at rifts and tion of potential crust and mantle sources, including plumes, principally tholeiitic to mildly alkaline basalt studies of basement outcrops and deep crustal and with high melting temperatures, high Fe/Mg, low f O2, mantle xenoliths. More isotopic systems must be low f H2O, high F/Cl, radiogenic Nd isotopic composi- brought to bear on the problem, including S, O, and tions, unradiogenic Sr isotopic compositions, and small Hf. For example, Goodge and Vervoort (2006) have or absent Nb–Ta–Ti–Pb anomalies. Such mafic mag- measured Hf isotopic compositions of zircons from mas are not common in orogenic settings where many Proterozoic A-type granites from Laurentia and magnesian, calc-alkalic magmas with low Fe/Mg, high found that they are indistinguishable from the crust they f O2, high f H2O, low F/Cl, low Sm/Nd, low concentra- intrude. Such Hf isotopic studies of young rhyolites and tions of incompatible elements, and pronounced Nb– granites are sorely needed, especially where there is a Ta–Ti–Pb anomalies are the rule (essentially calc-alka- large difference between the age of the basement and the line I-type granitoids). Most felsic continental crust is age of the anorogenic magmatism. Another fruitful formed in such orogenic settings and inherits many of avenue of research lies in better estimates of the volatile these characteristics and over time develops very low fugacities and oxidation states of A-type magmas: do 87 86 εNd values and high Sr/ Sr ratios. All of these they commonly crystallize at low f O2 consistent with characteristics are different from those of topaz rhyolites involvement of a reduced mantle component (e.g., Frost and rapakivi granites. Consequently, the most contro- and Frost, 1997) or is there a wide variation in f O2 versial conclusion of this paper is that felsic continental implied by the presence of magnetite-series A-type 242 E.H. Christiansen et al. / Lithos 97 (2007) 219–246 granites and titanite (e.g., Anderson and Morrison, Best, M.G., Christiansen, E.H., 1991. Limited extension during peak 2005; Dall'Agnol et al., 2005; Bogaerts et al., 2006)? Tertiary volcanism, Great Basin of Nevada and Utah. Journal of Geophysical Research 96, 13509–13528. Are f HF/f HCl ratios characteristically high in the Best, M.G., McKee, E.H., Damon, P.E., 1980. Space–time-composition parent magmas, and hence a clue to magma sources, patterns of late Cenozoic mafic volcanism, southwestern Utah and or are the high ratios a result of late degassing? adjoining areas. American Journal of Science 280, 1035–1050. In short, many questions still remain about the Best, M.G., Keith, J.D., Mehnert, H.H., 1987. Early Miocene ultimate sources of A-type granites. These questions can tectonism, magmatism, and mineralization in and near the southern Wah Wah Mountains, southwestern Utah. Oligocene and Miocene most fruitfully be addressed by comparative studies of Volcanic Rocks in the Central Pioche–Marysvale Igneous Belt, rhyolites and granites from multiple complexes through- Western Utah and Eastern Nevada. U.S. G.S. Professional Paper, out Earth's long history. pp. 29–47. 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